a) Field of the Invention
The invention relates to a method for separating lignin from black liquor, and more especially a black liquor pre-treatment method for reducing the acid requirements and improving the filterability of lignin slurries produced from the addition of acid to black liquors.
b) Description of Prior Art
It is well established in the prior art that, depending on the black liquor pH and the lignin molecular weight (MW), the lignin is either dissolved or dispersed in black liquor in the colloidal form with the colloidal dispersion being stabilized by charged phenolic and carboxylic acid groups on the lignin (Marton, J., On the structure of kraft lignin, Tappi, 47(11), 713-719 (1964). Each lignin colloid carries a negative charge which produces an electrostatic repulsion force between adjacent particles. If the charge is sufficiently high, the lignin colloids will remain discrete, disperse and in suspension. Reducing or eliminating the charge has the opposite effect leading to lignin particle agglomeration and settling out of solution (lignin precipitation). As a result, the stability of lignin solutions and/or dispersions can be affected by such factors as pH, ionic strength, temperature and surface-active agents (Norgren, M., Some aspects on the physical chemistry of kraft lignins in aqueous solutions. Theoretical considerations and practical implications, Lic. Thesis, Mid Sweden University, Sundsvall, Sweden, 2000; Norgren, M. and Edlund, H., Stabilization of kraft lignin solutions by surfactant additions, Colloids and Surfaces, A: Physicochemical and Engineering Aspects, 194 (1-3), 239,-248 (2001). As in other types of colloidal dispersions, lignin precipitation from black liquor is assumed to occur in two steps: nucleation and particle growth. It is the relative rates of these two steps that ultimately determine the size of the precipitated lignin particles. Over the last seventy years, several methods exploited the colloidal nature of lignin in black liquor to aggregate and coalesce it to larger particles thus facilitating its separation from black liquor through filtration. Such methods include: lignin precipitation by acidification to lower the black liquor pH, changing the ionic strength of lignin solutions using, for example, alcohols and/or calcium salts, addition of surface-active agents and membrane separation. Presently, lignin precipitation by acidification is the most common method used for lignin recovery. A number of such processes exist (e.g. Uloth, V. C. and Wearing, J. T., Kraft lignin recovery: acid precipitation versus ultrafiltration. Part I. Laboratory test results” Pulp & Paper Canada, 90(9), 67-71 (1989); Uloth, V. C. and Wearing, J. T., Kraft lignin recovery: acid precipitation versus ultrafiltration. Part II. Technology and economics” Pulp & Paper Canada, 90(10), 34-37 (1989); Loutfi, H., Blackwell, B. and Uloth, V., Lignin recovery from kraft black liquor: preliminary process design; Tappi Journal, 203-210, January 1991; Ohmann, F., Theliander, H., Tomani, P. and Axegard, P., Method for separating lignin from black liquor, WO2006/031175 A1). In most of these processes, the black liquor acidification is predominantly performed by using either carbon dioxide or a mineral acid (e.g. sulphuric acid) or a combination of the two to drop the pH of the black liquor from about 13-13.5 to 9-10. After acidification, the lignin is usually filtered and washed with acid (e.g. sulphuric acid) and water to produce lignin of high purity. In many cases, however, it is difficult to separate the lignin from the acidified black liquor solutions. In an effort to improve the filtration properties of acid-precipitated lignin slurries, a number of approaches have been considered in the prior art including filtration at high temperatures (80° C.-90° C.), increased ionic strength and reduced precipitation pH. Even though these approaches work to a certain extent, the filtration resistance is still quite high leading to unreasonably low filtration rates and, in certain cases, a lignin product of low dry solids content. This, in turn, leads to a large filtration area being required in the equipment needed for lignin filtration leading to high capital costs as well as increased drying costs for the lignin. A second problem associated with most lignin precipitation processes using acid is the large amount of acid (e.g. carbon dioxide and/or sulphuric acid) that is needed to induce the lignin to come out of solution and/or be converted from the sodium to the hydrogen form (e.g. during suspension of the lignin cake in an acid solution or the washing of the lignin with acid on the filter). A third problem associated with most lignin acid precipitation processes is the emission of totally reduced sulphur (TRS) compounds during most stages of the process. Such compounds which include hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide are strongly odorous compounds with well-known negative effects on human health and other forms of life.